Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic gasoline oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 700° C. to 1000° C.
A complete fuel cell assembly comprises a plurality of components and sub-assemblies joined together mechanically to provide the desired flow paths and control pathways for the liquid hydrocarbon, reactive gases, spent gases, and cooling gases. It is essential that the joints or interfaces between the components and sub-assemblies be durably leak-free at temperatures from below 0° C. to as high as 1000° C., at pressures from subatmospheric to up to several atmospheres. Such conditions place very high demands on materials selected for gaskets at these joints and interfaces.
It is known to use various glass and ceramic compositions as sealants. However, a drawback is that such sealants, though effective, tend to be quite brittle and are easily fractured in assembly or in use. Further, typically they require high-temperature sintering during manufacture of a fuel cell system, which adds difficulty and cost.
What is needed is a material for gasketing in an SOFC system which is thermally stable over the range between shutdown and operating temperatures for both the reformer and the fuel cell assembly; which is chemically stable in oxidizing and reducing environments; which is more robust during assembly and during operation of the system; which is compatible with other materials of the system; and which is relatively inexpensive.
It is a principle object of the present invention to provide an improved material for gasketing joints and seals in a fuel cell assembly.